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I looked at your capture and I saw that there was a retransmission of MLE Discovery Response, which means that the ACK wasn't transmitted back to K41Z board. But to be sure, run the application from NXP board in debug mode and put a breakpoint in "APP_JoinerDiscoveryRespCb" function from "app_thread_callbacks.c" file. If it was received, than that breakpoint should jump. If somehow it jumps, verify if the instruction "addToJoiningList = TRUE;" is executed.

Also, it would help me if you would send a capture in which the MLE Discovery Request is present, see the picture below.